Chapter 9: Nutrition and the Human Digestive System

9.1 Digestive System

• The alimentary canal runs from mouth to anus: mouth, oesophagus, stomach, small intestine, large intestine, anus; accessory organs are liver, gallbladder and pancreas. (p.144)

9.2 Digestion

• Physical digestion is mechanical breakdown (chewing, peristalsis); chemical digestion is enzyme-catalysed breakdown. (p.144)
• Saliva contains salivary amylase, which hydrolyses starch to maltose, at pH 6.5–7.5. (p.145)
• Peristalsis is rhythmic contraction and relaxation of muscles along the alimentary canal, pushing the bolus down the oesophagus. (p.145)
• Gastric glands: chief cells secrete pepsinogen, parietal cells secrete hydrochloric acid, mucous cells secrete mucus. (p.145)
• Pepsin (activated from pepsinogen by HCl) hydrolyses protein into polypeptides. (p.145)
• HCl provides pH 1.5–2.0 for pepsin, stops salivary amylase and kills bacteria; mucus protects the stomach wall. (p.146)
• Chyme (semifluid stomach contents) enters the duodenum when the sphincter relaxes. (p.146)
• Bile from the liver (stored in gallbladder) neutralises chyme, gives an alkaline medium (pH 7.6–8.6) and emulsifies lipids. (p.146)
• Pancreatic juice: pancreatic amylase (starch to maltose), trypsin (polypeptides to peptides), lipase (lipids to fatty acids and glycerol). (p.146)
• Ileum enzymes: maltase, sucrase, lactase (to monosaccharides), lipase and erepsin (peptides to amino acids). (p.147)
• Starch experiment: amylase with starch at 37°C; iodine test detects remaining starch and Benedict's test detects the reducing sugar formed. (p.148)
• Protein experiment: pepsin with albumen and HCl at 37°C; clarity of the mixture is compared with the control without pepsin. (p.149)
• Lipid experiment: lipase with oil, sodium carbonate and phenolphthalein; pink turns colourless as fatty acids form. (p.150)

9.3 Absorption

• Villus adaptations: one-cell-thick epithelium, blood capillary network, lacteal for fatty acids and glycerol, goblet cells secrete mucus. (p.151)
• The folded ileum with villi and microvilli gives a large surface area. (p.151)

Absorption in the ileum

(p.152)

Digested food	Method
Fructose	Facilitated diffusion into blood capillaries
Glucose, galactose	Active transport into blood capillaries
Amino acids	Active transport into blood capillaries
Vitamins B and C	Absorbed with water
Water	Osmosis
Fatty acids and glycerol	Recombine into lipid droplets; simple diffusion into lacteal
Vitamins A, D, E, K	Dissolve in lipid; simple diffusion

9.4 Assimilation

• Blood capillaries of the small intestine form the hepatic portal vein to the liver; lacteals join lymph vessels, then the thoracic duct into the left subclavian vein. (p.153)
• Liver: metabolises glucose and amino acids, stores excess glucose as glycogen, and detoxifies toxic substances. (p.153)
• Deamination converts excess amino acids to urea for excretion. (p.153)
• Amino acids are used to make protoplasm, repair tissue, hormones and enzymes; glucose is oxidised for energy or stored as glycogen in muscles; lipids build plasma membranes or are stored as fat. (p.154)

9.5 Defaecation

• The large intestine absorbs water and vitamins (B, K, folic acid from bacteria) and forms faeces. (p.155)
• Faeces move through the colon in 12–24 hours and accumulate in the rectum until defaecation. (p.155)

9.6 Balanced Diet

• Balanced diet contains all seven food classes: carbohydrates, lipids, proteins, vitamins, mineral salts, fibre and water. (p.156)
• Energy value (kJ g^-1) = water mass × 4.2 × temperature rise / (mass of food × 1000). (p.156)
• Vitamin C is tested with DCPIP; the volume of juice needed to decolourise DCPIP indicates content. (p.158)
• Low temperature preserves vitamin C in orange juice. (p.159)
• Obesity results from storing excess fat; reduce carbohydrates and fats and increase fruit and vegetables. (p.160)
• Excess saturated fat and cholesterol can cause diabetes mellitus and cardiovascular diseases. (p.160)
• Cancer patients modify diets to get enough energy and reduce infection risk. (p.160)

9.7 Health Issues Related to Digestive System and Eating Habits

• Gastric bypass reduces stomach size; side effects include acid reflux, nausea, malnutrition and ulcers. (p.161)
• Fibre prevents constipation, colon and rectum cancer and haemorrhoids; stimulates peristalsis. (p.161)
• Gastritis: inflammation of the stomach epithelium from irregular meals, alcohol or painkillers. (p.162)
• Anorexia nervosa: avoiding food; bulimia nervosa: binge eating then vomiting or laxatives; muscle dysmorphia: extreme weightlifting and steroid use. (p.162)